Subject: ARM: KVM/XEN: how should we support virt-what?

From: [email-masked] (Andrew Jones)
Date: Thu, 26 Mar 2015 10:01:51 +0100

On Wed, Mar 25, 2015 at 10:44:42AM +0100, Andrew Jones wrote:
> Hello ARM virt maintainers,
> 
> I'd like to start a discussion about supporting virt-what[1]. virt-what
> allows userspace to determine if the system it's running on is running
> in a guest, and of what type (KVM, Xen, etc.). Despite it being a best
> effort tool, see the Caveat emptor in [1], it has become quite a useful
> tool, and is showing up in different places, such as OpenStack. If you
> look at the code[2], specifically [3], then you'll see how it works on
> x86, which is to use the dedicated hypervisor cpuid leaves. I'm
> wondering what equivalent we have, or can develop, for arm.
> Here are some thoughts;
> 0) there's already something we can use, and I just need to be told
>    about it.
> 1) be as similar as possible to x86 by dedicating some currently
>    undefined sysreg bits. This would take buy-in from lots of parties,
>    so is not likely the way to go.
> 2) create a specific DT node that will get exposed through sysfs, or
>    somewhere.
> 3) same as (2), but just use the nodes currently in mach-virt's DT
>    as the indication we're a guest. This would just be a heuristic,
>    i.e. "have virtio mmio" && psci.method == hvc, or something,
>    and we'd still need a way to know if we're kvm vs. xen vs. ??.
> 
> Thanks,
> drew 
> 
> [1] http://people.redhat.com/~rjones/virt-what/
> [2] http://git.annexia.org/?p=virt-what.git;a=summary
> [3] http://git.annexia.org/?p=virt-what.git;a=blob_plain;f=virt-what-cpuid-helper.c;hb=HEAD

Thanks everyone for their responses. So, the current summary seems to
be;
1) Xen has both a DT node and an ACPI table, virt-what can learn how
   to probe those.
2) We don't have anything yet for KVM, and we're reluctant to create a
   specific DT node. Anyway, we'd still need to address ACPI booted
   guests some other way.

For a short-term, DT-only, approach we could go with a heuristic, one
that includes Marc's "if hypervisor node exists, then xen, else kvm"
condition.

How about SMBIOS for a long-term solution that works for both DT and
ACPI? We're not populating SMBIOS for arm guests yet in qemu, but now
that AAVMF has fw_cfg, we should be able to. On x86 we already have
smbios populated from qemu, although not in a way that allows us to
determine kvm vs. xen vs. tcg.

Any other suggestions?

drew